FIG. 11 is a circuit diagram illustrating the configuration of a conventional multiphase current supplying circuit. A power supply system 1 includes a single-phase or multiphase, e.g., three-phase ac power supply 13, and supplies an ac voltage Vin to an AC-DC converter (hereinafter briefly referred to as a “converter”) 2. An inductance parasitic to the power supply system 1 is shown as an inductor 12 connected in series to the ac power supply 13.
An intervening circuit 3 is interposed between the converter 2 and an inverter 4, and the output of the converter 2 is supplied to the intervening circuit 3. The intervening circuit 3 includes a capacitor 31, and the output of the converter 2 is supplied to the both ends of the capacitor 31. The capacitance C of the capacitor 31 is small, and selectively set at, e.g., 20 μF. The capacitor 31 can be reduced in size by decreasing its capacitance C.
A rectified voltage Vdc which is a both-end voltage of the capacitor 31 is input to the inverter 4. In the inverter 4, switching of transistors serving as its switching devices of the inverter 4 is carried out on the basis of switching signals Tu, Tv and Tw obtained from a control circuit 6. As a result, three-phase currents iu, iv, iw are thereby supplied to a motor 5.
The control circuit 6 is supplied with a phase θ1 of the ac voltage Vin, rectified voltage Vdc, currents iu, iv, iw, and a rotation position angle θm of a rotor of the motor 5. These respective quantities can be detected using a well-known technique. On the basis of these quantities, the control circuit 6 generates the switching signals Tu, Tv and Tw.
A technique is publicly known which extremely reduces the capacitance C of the capacitor 31 and appropriately controls the switching signals Tu, Tv and Tw on the basis of the above-mentioned respective quantities, to thereby carry out AC-AC conversion. Such switching control will herein be called capacitorless inverter control. The capacitorless inverter control allows size reduction of the whole circuit including the capacitor and inverter to achieve cost reduction, as compared with an ordinary circuit with the intervening circuit 3 replaced by smoothing circuit 301 or 302 (shown in FIGS. 12 and 13, respectively). While the smoothing circuit 301 employs a smoothing large-capacitance capacitor CC and a power factor correction reactor LL, the capacitorless inverter control can suppress a reduction in power factor on the power supply side without using such power factor correction reactor LL. In the case of using a single-phase power supply, the smoothing circuit 302 is further provided with a diode DD and a transistor QQ serving as a switching device to constitute a chopper circuit in order to reduce higher harmonics of the power supply, however, the capacitorless inverter control can suppress higher harmonics of the power supply without using the chopper circuit.
Single-phase capacitorless inverter control is disclosed in, for example, Non-patent document 1. In Non-patent document 1, a rectified voltage which greatly pulsates at a frequency almost twice that of a single-phase ac power supply is applied to an inverter, but a three-phase ac current is output by appropriate control of switching in the inverter. Non-patent document 1 shows that, in the single-phase capacitorless inverter control, the power factor takes an excellent value of 97% or higher where the maximum value of the both-end voltage of a capacitor is not lower than twice the minimum value thereof.
Three-phase capacitorless inverter control is disclosed in, for example, Non-patent document 2. In Non-patent document 2, a rectified voltage which pulsates at a frequency six times that of a three-phase ac power supply is applied to an inverter, but a three-phase ac current is output by appropriate control of switching in the inverter. Non-patent document 2 shows that, in the three-phase capacitorless inverter control, the power factor takes an excellent value of 95.5% or higher where the minimum value of the both-end voltage of a capacitor is not higher than 31/2/2 times the maximum value thereof.
Further, Non-patent document 3 discloses capacitorless inverter control having a three-phase active converter. Non-patent document 3 shows that appropriate control of switching of the active converter can stabilize the both-end voltage of a capacitor, and further, can suppress higher harmonics of a power supply.
Non-patent document 1: Isao Takahashi “Inverter Controlling Method for a PM Motor having a Diode Rectifying Circuit with a High Input Power Factor” The Institute of Electrical Engineers of Japan, National Conference in 2000, 4-149 (March 2000), p. 1591
Non-patent document 2: Yoichi Ito, Isao Takahashi “Capacitorless PWM inverter” 1988, The Institute of Electrical Engineers of Japan, Industry Applications Society, National Conference, pp. 445-450
Non-patent document 3: Yoichi Ito, Isao Takahashi, Fumiaki Hachiboshi, Kazuhiko Tanaka “Capacitorless PWM inverter (Study on PWM Control Technique)” 1989, The Institute of Electrical Engineers of Japan, National Conference, pp. 5-89 to 5-90.
In the power supply system 1 of the multiphase current supplying circuit employing the capacitorless inverter control as described above, a case where a lightning surge is superimposed is assumed. Therefore, it is desirable to take measures for lightning protection in the power supply system 1.
FIG. 14 is a circuit diagram showing the configuration where a lightning arrester 7 is interposed between the power supply system 1 and converter 2 in the multiphase current supplying circuit shown in FIG. 11. The converter 2 receives the ac voltage Vin via the lightning arrester 7. Here, the lightning arrester 7 serves as a peak-value suppressor for suppressing a surge voltage superimposed on the ac voltage Vin.
Damage that the inverter 4 receives when a lightning surge is superimposed in the power supply system 1 will be considered. FIG. 15 is a graph showing a waveform 101 of the ac voltage Vin and a waveform 110 of the rectified voltage Vdc. Herein, simulation was run in the case where one phase of the ac power supply 13 (when the ac power supply 13 is a single-phase ac power supply, its output) generated a sinusoidal voltage having a frequency of 50 Hz and an effective value of 270 V, and a lightning surge of several thousands of volts with a width of 50 μs occurred near the peak of the sinusoidal voltage. While an inductance L0 of the parasitic inductor 12 may actually vary by regional power distribution conditions (the lengths of power lines and difference in leakage inductance of transformers), 230 μH was employed in this simulation. For the capacitance C of the capacitor 31, 20 μF was employed. The ac voltage Vin was assumed to be clamped at 800 V by the lightning arrester 7. For simplification, the simulation was run in the case where the inverter 4 was on standby (when an active converter is provided, the active converter, too), and the motor 5 was not supplied with current (iu=iv=iw=0).
The waveform 110 of the rectified voltage Vdc almost coincided with the peak value (20.5×270V) of the ac voltage Vin until immediately before superposition of the lightning surge, but increased by slightly over 250 V after the superimposition, and the peak value exceeded 600 V. Shown is the simulation where the motor 5 was not supplied with current (when the inverter 4 was on standby, for example), the waveform with the rectified voltage Vdc maintained in magnitude is shown. However, this problem about the peak value occurs even when the motor 5 is supplied with current.
In the case where the power supply is a 200V system, components having a breakdown voltage of about 600 V are often selected as the transistor to be used in the inverter circuit 4 for the purpose of its size reduction. Accordingly, there is a high possibility that the superimposition of a lightning surge, even when the lightning arrester 7 reduces the value, on the ac voltage Vin as shown in FIG. 15 may cause serious damage on the inverter circuit 4.
Such phenomenon, however, does not cause great damage on the inverter circuit 4 in the case where the capacitance C of the capacitor 31 is large. FIG. 16 is a graph showing the waveform 101 of the ac voltage Vin and a waveform 111 of the rectified voltage Vdc. The graph of FIG. 16 shows the results of simulation employing 900 μF for the capacitance C of the capacitor 31, unlike in the graph of FIG. 15. In this case, the rectified voltage Vdc rose to as low as four hundred and several tens of volts while the ac voltage Vin reached as high as 800 V.
This is considered because, as the capacitance C decreases, a charging current ic flown into the capacitor 31 via the converter 2 by the lightning surge causes a higher voltage to be generated at the capacitor 31. In other words, to carry out the capacitorless inverter control having the above-described advantages, a voltage rise in the capacitor 31 due to the lightning surge needs to be suppressed.